1. Field of the Invention
The present invention describes optical fibers, and more particularly a doped optical fiber useful as a pump laser pigtail fiber in an optical amplifier.
2. Description of the Related Art
Er3+-doped fiber amplifiers have revolutionized optical telecommunications by providing all-optical high-gain, low-noise amplification over many channels without the need for costly electronic repeaters. As the optical path in amplifiers and other optical devices becomes more complex, suppressing reflections off of various components such as, for example, laser diode facets, filters, variable optical attenuators, becomes increasingly difficult. These reflections can be deleterious to amplifier performance. For example, in a process known as multi-path interference (MPI), stray light of wavelengths in the signal band can reflect off of diode facets, be fed back into the amplifier coil and amplified, and interfere with the single pass signal. This contributes noise to the system and degrades the performance of the amplifier. Thus, in current amplifier designs it is preferable that reflections in the signal band from amplifier components, such as pump lasers, be minimized in order to reduce the detrimental effects of multi-path interference (MPI). These specifications are often quite strict, sometimes requiring as much as xe2x88x9217 dB suppression of signal band reflections. Existing methods for meeting this specification are either complex, costly, or produce additional reliability risk.
For example, in one method of MPI suppression pump manufacturers may use anti-reflection coatings in the signal band on the pump laser facet. This design requires a complex coating, since the facet reflectivity must be controlled at both 980 nm for correct pump operation, and around 1550 nm for MPI suppression. Anti-reflection coatings, however, are expensive, and it is often not possible to deposit coatings sufficiently on some surfaces such as pump diode facets, which are required to have high reflectivity or partial transmission at the pump wavelength while also providing facet passivation.
In other configurations, signal band loss in the pump laser fiber pigtail may also be increased by bending the pigtail fiber appropriately around fairly small mandrels. By using appropriately sized mandrels, bend loss for the longer wavelength signal band will be greater than that for the pump wavelength. This design places undesirable restrictions on the amplifier designs, increases reliability risks, and does not work with all fiber profiles.
In an alternative MPI suppression technique, precision positioning of the fiber interface surface with respect to the laser diode facet and the appropriate fiber-surface reflectivity can be used to produce destructive interference of the reflected signal, but this again is costly and requires that only minimal positioning shifts (e.g. less than 300 nm) occur over the lifetime of the pump laser module.
It is thus desirable to have an optical fiber suitable for use as a pigtail for a diode pump laser that can provide MPI suppression in an optical amplifier without coiling, anti-reflection coating, or precision positioning.
One aspect of the present invention is a selectively absorbing optical fiber for use as a filter which is transparent at wavelengths in a pump band and highly absorbing at wavelengths in a signal band. For use herein, transparent is defined as having an absorption loss of less than about 0.5 dB/m, and highly absorbing is defined as having an absorption loss of greater than about 4.0 dB/m. For example, selectively absorbing optical fibers of the present invention can have an absorption loss of less than about 0.5 dB/m, less than about 0.2 dB/m, or even less than about 0.05 dB/m at wavelengths of in a pump band; and an absorption loss of greater than about 2.0 dB/m, greater than about 4.0 dB/m, or even greater than 5.0 dB/m at wavelengths in a signal band. The pump band may include, for example, wavelengths between 970 and 990 nm, and the signal band may include, for example, wavelengths between 1500 and 1600 nm.
Another aspect of the present invention is a selectively absorbing optical fiber having a core and a cladding layer, wherein at least one of the core layer and the cladding layer includes a selectively absorbing composition; and wherein the selectively absorbing optical fiber has an absorption loss of less than 0.5 dB/m at wavelengths in a pump band, and an absorption loss of greater than 2.0 dB/m at wavelengths in a signal band; and wherein the selectively absorbing optical fiber is used as a filter. The selectively absorbing composition may include a rare earth element. The rare earth element may be, for example, selected from the group consisting of thulium, praseodymium, neodymium, terbium, samarium, and dysprosium. The pump band may include, for example, wavelengths between 970 and 990 nm, and the signal band may include, for example, wavelengths between 1500 and 1600 nm. The selectively absorbing optical fibers of the present invention may be polarization maintaining and/or photosensitive, and may be based on silica, germanosilicate, borosilicate, or aluminosilicate materials.
Another aspect of the present invention is an optical device including a selectively absorbing optical fiber including a core and a cladding layer, wherein at least one of the core layer and the cladding layer includes a selectively absorbing composition; and wherein the selectively absorbing optical fiber has an absorption loss of less than 0.5 dB/m at wavelengths in a pump band, and an absorption loss of greater than 2.0 dB/m at wavelengths in a signal band; and wherein the selectively absorbing optical fiber is used as a filter. The selectively absorbing composition may include a rare earth element. The rare earth element may be, for example, selected from the group consisting of thulium, praseodymium, neodymium, terbium, samarium, and dysprosium. The optical device may include a laser with a pigtail, and the pigtail may include a selectively absorbing optical fiber of the present invention. The optical device may be an optical amplifier, with the selectively absorbing optical fiber of the present invention in the optical path between a pump laser and an amplifying fiber.
Another aspect of the present invention is a method of suppressing multi-path interference due to stray reflected signal in an optical amplifier, the optical amplifier having a pump laser, an amplifying fiber, and an optical path, the method including the steps of including in the optical path a length of selectively absorbing optical fiber, and absorbing the stray reflected signal with the selectively absorbing optical fiber.
The present invention results in a number of advantages over the prior art by providing an optical fiber that is highly absorbent at wavelengths in the signal band (e.g. wavelengths between 1500 and 1600 nm) but not at wavelengths in the pump band (e.g. 970-990). Such a fiber will absorb any stray unwanted reflected signal while permitting the pump energy to pass, thus suppressing multi-path interference. Use of this fiber in a pump laser pigtail adds little to no complexity to the laser manufacturing process and, in an amplifier, decreases the need for costly manufacturing processes such as anti-reflective coating and/or precision positioning control.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the invention as described in the written description and claims hereof, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework to understanding the nature and character of the invention as it is claimed: